The present invention is directed to methods and apparatus for providing receivers for use in superposition coded multi-user systems, and more particularly to extracting data from a superposition coded signal using an enhanced log-likelihood ratio (LLR) computation circuit.
Wireless communication systems operating with multiple users generally include a base station (or a transmission source) which broadcasts data signals to all users (referred to as downlink) and which receives data signals from each of the users (referred to as uplink). As defined herein, the term user represents any device remote from the transmission source or base station including MP3 players, portable computing devices, personal digital assistants, laptops, mobile phones, wireless devices, or any other device with wireless communication capabilities that can receive or transmit superposition coded signals. Each user must have the capability of differentiating the user's data from the data corresponding to the other users which is present in the same broadcasted signal. Similarly, the base station must have the capability of differentiating each user's data since it receives all of the data as one signal from all of the users. Thus, an orthogonal design is typically used which either separates each user's data into a different time slot (time-division multiple access), a different frequency slot (frequency division multiple access) or a different coding scheme (code division multiple access).
Although such orthogonal systems are relatively simple to implement and provide high data reliability, the data rate achieved is not optimal. Superposition coding systems increase the data rate by simultaneously providing the data signals for all of the users in at least one of the same time slot, frequency slot and coding scheme. However, the receivers in superposition coding systems are more complex than those used in orthogonal systems and thus impractical to implement.
More specifically, the receivers in superposition coding systems typically apply high complexity successive interference cancellation (SIC) algorithms to extract the data corresponding to a particular user. SIC algorithms have a high complexity because a particular user's receiver has to decode every other user's data first and cancel from the received signal the other user's data before the user's receiver extracts the user's own data from the received signal. As defined herein extracting data means computing an information (data) bit stream containing information about the reliability of each received symbol (i.e., soft decoding).
Accordingly, low complexity receivers are critical to obtain high data rates with greater data reliability in superposition coded multi-user systems.